Research On High-temperature Oxidation Behaviors Of Zr-Al Alloys

The oxide layer formed on the surfaces of metals during thermal oxidation has strong influence on the surficial properties of the metals,such as corrosion resistivity,friction,wear,catalytic properties and long-term stability/reliability.Zr-Al intermetallic alloys have drawn extensive attention in recent years,due to their low neutron-capture cross-sections,high corrosion resistance,low density,excellent thermal stability and high tensile strength.Therefore,Zr-Al alloys have promising applications in wide areas,for example,as fuel cladding materials for nuclear reactor and hydrogen getters in vacuum systems.Among various Zr-Al compounds(e.g.ZrAl₃,ZrAl₂,Zr₂Al₃,ZrAl,Zr₅Al₄,Zr₄Al₃,Zr₃Al₂,Zr₅Al₃,Zr₂Al and Zr₃Al),ZrAl₂ possesses the most outstanding physical properties,such as the largest elastic modulus,the strongest average bond strength of atoms and the highest hardness.In this dissertation,the thermal oxidation behavior of ZrAl₂ alloy at different temperatures was systematically studied.The mechanism of ZrAl₂ alloy forming a micron thick amorphous oxide layer at high temperature was revealed.The evolution mechanism of the oxide layer structure was clarified.The main research findings are as follows:The thermal oxidation leads to the growth of an amorphous(Zr_0.33Al_0.67)O_1.66layers with a thickness of micron(up to 4.5?m)at the ZrAl₂ surface in the temperature range of 550-750?.The oxidation kinetics follows a parabolic law with an activation energy of 143 k J/mol.The underlying mechanism for the formation of such micrometer-thick amorphous oxide surficial layers has been discussed on the basis of(i)interface thermodynamics and(ii)the occurrence of high interface stability associated with a synchronous oxidation of Al and Zr elements.As the oxidation temperature increases from 800? to 950?,the oxide layer structure formed on the surface of ZrAl₂ transforms from an amorphous structure to a crystalline structure,and a multilayered crystalline oxide structure is formed.The oxidation kinetics follows a parabolic law in the temperature range of 800-950?.The activation energy was determined to be 239 k J/mol in the temperature range of 800-950?.The transition temperature of the ZrAl₂ from uniform oxidation to selective oxidation is determined to be 850?.The mechanism for transformation of the oxide layer structure from single-layer to multilayer structure are as follows.i)The crystallization of the amorphous oxide layer results in uniform oxidation in the early oxidation stages.ii)As the oxidation proceeds,the oxide layer transforms into a multilayer structure due to the competitive effect of both the high oxygen affinity of Zr and the concentration instability in front of the oxidation-reacting region.